1. Field of the Invention
This invention relates to a belt filter apparatus. In particular the invention concerns a belt filter apparatus for use in drying fine material, typically fine coal. The invention also extends to a method of drying fine material.
2. Description of the Related Art
Coal which is mined from the ground often contains unwanted impurities such as carbonaceous and non-carbonaceous material which are reduced to various different sized fragments. Users of coal, however, normally require coal of consistent quality. For this reason mined coal should undergo a process referred to as beneficiation, also known as coal preparation or coal washing. Coal beneficiation refers to the treatment of coal to ensure consistent quality and to enhance its suitability for specific end-users.
The treatment of coal will be dependent upon the physical properties of the coal and its intended purpose. For certain applications the treatment may simply require crushing while in other instances it may be necessary for the coal to undergo a complex process for reducing impurities.
In order to remove impurities, coal is crushed and thereafter separated into various coal fractions, whereafter the fractions typically undergo a further treatment referred to as dense medium separation. In this process, the coal is separated from impurities by being floated in a tank containing a liquid of specific gravity, usually water holding finely ground magnetite in suspension. Due to the fact that the coal is less dense than the liquid, it will float and can be separated off while heavier rock and other impurities sink and can be removed as waste.
The process of beneficiation typically generates a fine fraction referred to as slurry, a slightly coarser fraction which is known as duff and an even coarser fraction known as discard. In order for the fine fractions to be saleable the moisture would need to be removed from the fine fractions. In a South African context water is a scarce commodity and the use thereof consequently strictly controlled by the South African Department of Water Affairs and Forestry. It will, therefore, be appreciated that coal should be separated from the water in the slurry and duff in such a manner that the water can be re-used in future beneficiation processes. The mining industry has for this reason developed various methods for drying fine and ultra fine materials. These methods typically utilise drying beds, belt filter presses and hydraulic filter presses. A brief description of the mentioned methods is provided below together with advantages and drawbacks associated with the methods.
Drying beds are normally provided by constructing lined, large surface area dams into which slurry can be placed. The water can hereafter be removed from the drying beds with the use of for example, penstock systems. Water is decanted from the surface of the slurry, the so-called supernatant water, and once the drying beds are dry the fine material is mechanically excavated and sold. Making use of drying beds has the advantage that once the drying beds are in place, drying costs are minimal while the drying beds can handle large volumes of slurry. The drawbacks of making use of drying beds include a substantial capital outlay for constructing such drying beds, loss of water due to evaporation, loss of income due to delays during the period of drying, a potential for environmental spills due to over-topping and chemical breakdown of coal due to prolonged exposure to oxidation.
A belt filter press typically comprises two belts between which slurry can be placed and thereafter pressed. A belt filter also makes use of a vacuum for sucking water from the filter. The advantages of using a belt filter press includes that continued drying can take place, a relative small operating area is required and water that is removed from slurry can be recovered immediately. Further advantages include that the dried product can be added to the product line and a relatively quick turnaround of revenue is possible. The drawbacks of using belt filter presses include high installation costs, high material and maintenance costs and consequential high running costs.
A very important drawback of making use of a belt filter press is that by compressing hydraulically placed material, the permeability of such material is reduced with a consequential reduction in the ability of extracting all free water. Compressing the water containing material only serves to increase the pore pressure of the material with the consequential result that the largest volume of water is extracted prior to compaction. Energy which is therefore used with a view of removing free water is in fact wasted.
A hydraulic filter press comprises a bag in which fine material can be placed whereafter the material in the bag can be compressed hydraulically. The hydraulic filter press also makes use of a vacuum for sucking water from the bag. Upon completion of a drying cycle, batch material is blown from the bag into a bin and the cycle repeated. Advantages of making use of a hydraulic filter press include fast recovery of water, a minimal potential for detrimental environmental impact and fast realisation of a saleable product. The drawbacks of hydraulic filter presses, however, include high installation costs, high running and maintenance costs as well as batch processing, i.e. there is no continuous flow of material and consequently large volumes of liquid should be stored. Making use of hydraulic filter presses also results in compressed materials with increased pore pressure and reduced permeabilities similar to the situation with belt filter presses.
It is an object of the present invention to provide an alternative apparatus and method for use in drying coal slurry and which addresses the drawbacks of existing technology.